This invention relates to the storage of audio information on compact disks, and more specifically, to augmenting the standard, stereo compact disk with additional audio information, such as for higher resolution or multi-channel sound.
The compact disk (CD) has become the primary source for the delivery of recorded music due to its advantages over other media previously available to the consumer. It is of relatively small size and requires little special handling. As it is digitally recorded, it is subject neither to surface noise nor wear during playback.
The CD also has a number of disadvantages and limitations. Some of these inherent in nature of digital audio: Whenever music or other audio data is digitized, a certain amount of information is necessarily lost. Although this can be minimized by increasing the sampling rate, the number bits per sample, or both, there will still be some unavoidable loss. Although when a master recording is made digitally it usually employs this sort of higher resolution, when the actual CD itself is produced it must conform to the lower standards found in the accepted consumer format. For this reason, many audiophiles prefer to use analog vinyl recordings despite their surface noise when played, their resultant wear, and their more delicate handling and equipment requirements.
Another limitation imposed by the accepted standard for the CD is that of two channel, stereo sound. Within motion picture soundtracks and video games, multi-channel surround sound has become common, whether through having more than two speakers (such as for 5.1 channel or other cinema techniques), or through just two speakers or headphones by use of well know spatialization techniques utilizing delay, head related transfer function, and so on. To place such multi-channel sound onto a two channel disk requires the initial multi-channel sound to be encoded into two channels for recording, and then decoded back to a multi-channel signal for playback. For example, one set of standard encoding (or matrixing) methods encodes, say, three initial sound channels down to two channels, which are then recorded onto the CD or other stereo media, and then decodes this back to three channels upon playback, an arrangement known as 3:2:3 matrix sound . However, as the intermediate recording is required to be playable in its stereo form (or back-compatible), some information is again necessarily lost as part of this process.
One way around these shortcomings is to redesign the way data is stored on the compact disk: A higher sampling rate and more bits per sample would increase resolution; formatting the disk for more channels would allow unencoded surround sound. However, any such change would not conform to the accepted standard, the “Red Book”, for CD audio. The very success of the current CD format makes either the introduction of a non-conforming CD, that would not be back-compatible with current players, or, conversely, the introduction of a player incapable of reproducing a standard CD an unlikely option.
To allow for the inclusion of additional audio information within the standard CD audio tracks, while still maintaining back comparability with existing systems, the prior art has presented several techniques, both for encoding multiple channels and for improving resolution. As noted above, a number of matrixing techniques are know for encoding m-channels onto the standard two channels, and then decoding this out to n-channels on playback. However, for any of these m:2:n matrixing techniques, if the intermediary, stereo stage is to be back-compatible, the encoded two channels are limited a pair of linear combinations of the m input signals. As no complete set of functions can formed in this way for m>2, information is lost. Through proper mixing and use of decoding algorithms, these techniques can be successfully used for cinematic effects, but will be deficient for broader audio applications.
For improving resolution while maintaining back-compatibility, some prior art methods have placed additional audio information within the conventional signal by, in essence, hiding it. One set of techniques relies upon the “masking effect”, a psycho-acoustic effect whereby this additional data is encoded within the standard stereo signals, but in a way to make it relatively imperceptible if the CD is reproduced on a standard player. When played on a special player, however, the additional data can be decoded. This has several limitations: A first is that the requirement that the additional information is to remain relatively inaudible during normal playback limits the amount of additional data that may be encoded. Therefore, there is a limit to how much the resolution may be improved. A second, related limitation is that although the purpose is to improve the resolution upon playback, the standard, unencoded signal must be denigrated to accommodate the sub-audible information. Thus, a trade off must be made between the quality of the decoded signal and the signal available from a standard CD player.
It has also been suggested that additional audio information for improving resolution can be hidden in the subcode. The subcode is the portion of the CD which instructs the player on how to reconstruct the audio output based on the digitized recording. However, the amount of unused or redundant space available within the subcode is quite limited, greatly restricting the utility of this technique.
Aside from their original audio application, CDs also find use in CD-ROM applications. When used as a CD-ROM, part or all of the CD contains data formatted as a ROM memory that is read by a computer through a random access CD-ROM drive. In its more general form, a CD contains an independent audio portion, which is structured as a standard stereo music CD and is playable on a standard CD player, in addition to one or more CD-ROM sectors formatted as computer files, which are not accessible with a standard CD player. In some applications, such as computer games stored on a CD-ROM, the CD-ROM portion contains the music reproduced while the game is played. Since this music is inaccessible with a standard CD player, it is common to place a second, independent copy of this music in the audio portion to allow it to be listened to with a standard CD player. As such, this second copy is structured as a standard stereo CD audio recording and, accordingly, suffers from the same limitations of resolution and restriction to two channels already described. Additionally, as the volume of a CD is limited, storing a second, independent copy of the music in the audio portion is done at the expense of the volume available to the CD-ROM portion.